In general, a hydraulic construction machine controls the flow rate of a hydraulic pump in accordance with the operation rate of an operation lever (which means pilot signal pressure that is supplied to a spool in proportion to the operation amount of the operation lever to shift the spool that controls the flow of hydraulic fluid) in order to save energy.
In the related art, the relationship between the operation rate and the discharge flow rate of a hydraulic pump is constant regardless of load pressure. That is, in the case of controlling the discharge flow rate regardless of the load pressure, a large amount of hydraulic fluid is discharged from the hydraulic pump even when middle or high load is generated, and thus a loss of the flow rate and pressure occurs to cause the occurrence of energy loss.
On the other hand, during working using an excavator or the like, hydraulic flow of a desired flow rate is discharged in proportion to the operation rate of the operation lever in the case where work is done with load pressure that is lower than a standard load pressure at which the range of change of the discharge flow rate is wide and an accurate control is required.
By contrast, during working with load pressure that is higher than the standard load pressure, that is, in the case of lifting and moving a heavy object slowly, a high flow rate is unnecessary and the change of the flow rate is not great. During excavating and carrying work, the operation rate rapidly reaches the maximum level. Accordingly, during working with load pressure that is higher than the standard load pressure, the correction of the relationship between the operation rate and the discharge flow rate according to the load pressure do not cause a great change in operation feeling.